A resistor in an ac circuit behaves as it does in a dc one. It opposes the flow of current.The higher the resistance, the lower the current.
The higher the voltage across the resistor, the higher the current through it.
We can apply Ohm's Law.
Voltage and current must both be rms or peak, not a mixture of the two.
Inductive Reactance |
The coil opposes the flow of ac current, as a resistor does in a dc circuit. This opposition is called inductive reactance, XL. It is measured in ohms. Ohm's Law can be applied, as in the top formula.
The bottom formula shows how inductive reactance is calculated.
f is the frequency of the applied voltage, and L is the value of the coil in Henries.
It can be seen from this formula that the value of XL goes up as the frequency increases. It also goes up if the value of the coil increases.
This means that as the value of L or f increases, the opposition to the flow of ac current increases, and the lamp will glow less.
Capacitive in an AC Circuit |
The capacitor opposes the flow of ac current, as a resistor does in a dc circuit.This opposition is called capacitive reactance, Xc. It is measured in ohms. Ohm's Law can be applied, as in the top formula.
The bottom formula shows how capacitive reactance is calculated.
f is the frequency of the applied voltage, and C is the value of the capacitor in Farads.
It can be seen from this formula that the value of Xc goes down as the frequency increases. It also goes down if the value of the capacitor increases.
This means that as the value of C or f increases, the opposition to the flow of ac current decreases,
R, C and L in an AC Circuit |
The resistor, the capacitor and the coil all oppose the flow of ac current.Their combined opposition is called impedance, Z.
Ohm's Law can be applied, as shown in the top formula.
The higher the impedance, the lower the current.
The resistor has resistance. The capacitor has capacitive reactance. The coil has inductive reactance.
All of these values are measured in ohms.
However, impedance is not calculated by adding these values.
The bottom formula must be used.
If you are familiar with Pythagoras, this is another application of it.
When coupling one device to another, such as an amplifier to a loudspeaker, the output impedance of the amplifier and the input impedance of the speaker must be the same, to give optimum transfer of power from one to the other. |
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